Recently, the use of sulfur-containing fuels such as heavy oil, etc. has remarkably increased, so that when they are combusted harmful sulfur oxides such as SO.sub.2, SO.sub.3, etc. are formed in the exhaust combustion gas, presenting a severe social problem on public nuisance, and therefore, the development of an efficient process for their removal is being earnestly desired. As the wet processes for desulfurization of exhaust gases aimed at the prevention of public nuisance there have been proposed ammonia process, lime process, soda process, etc., and fairly most of them have already reached the stage of practical utilization, but all of them have their merits and demerits. Since the quantity of sulfur recovered for the removal of sulfur oxides is enormous, it can readily be imagined that with full operation of the desulfurization process the sulfur by-product will rapidly run into an over-supply and in future its supply-demand balance will eventually collapse. Thus, it becomes necessary that the sulfur compound is recovered in the form suited to mass consumption as well as to safe disposal. In this regard it is considered that gypsum is one of the products meeting such requirements. Gypsum has many advantages such that it is much in demand, harmless, incombustible, and chemically stable, and it can be disposed as waste material in land reclamation without the danger of secondary public nuisance, and moreover, the lime used as its neutralizing agent is not only exceptionally cheap as compared with other neutralizing agents, but also readily available in a long-lasting steady state.